Paged column dictionary

ABSTRACT

A request for an entry of a dictionary having multiple dictionary blocks may be received, where the dictionary stores string values associated with corresponding value identifiers (IDs) for dictionary compression. One of the dictionary blocks may be selectively loaded into memory, and the dictionary block that has been loaded into memory may be searched. A value ID directory may be constructed in memory, where the value ID directory includes last value IDs for the dictionary blocks, and each of the last value IDs is mapped to an index of one of the dictionary blocks that includes a string value for that last value ID. A separator directory may also be constructed in memory, where the separator directory includes separators for the dictionary blocks, and each of the separators is mapped to an index of one of the dictionary blocks that includes a string value for that separator.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to and incorporates by reference, U.S. patent application Ser. No. 14/333,224, filed Jul. 16, 2014, which in turn claims the benefit of U.S. Provisional Application No. 61/907,296, filed Nov. 21, 2013, the disclosure of which is hereby incorporated by reference.

BACKGROUND

A database is an organized collection of data. Conceptually, data in a database can be organized as one or more tables, where a table is a two-dimensional structure with data values organized in rows and columns. A row of a table contains the data values for one record of the table. A column of the table contains the data values of one field of the table across multiple records (rows) of the table.

An in-memory database may be loaded in main memory instead of read from disk storage as for some database systems. In-memory databases have the advantage of being able to be read from and written to quickly since read and write speeds for main memory are almost always faster than for disk storage. However, as database systems get larger, it becomes increasingly difficult to store the database (or tables of the database) entirely in main memory.

Therefore, there exists ample opportunity for improvement in technologies related to managing in-memory database systems.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Techniques and solutions are provided for implementing paged column dictionaries within in-memory database systems. In many scenarios, the memory footprint of tables with a large number of columns and/or large columns can be reduced by supporting paged columns such that dictionaries are paged using traditional buffer cache and paging technologies.

In some embodiments, a request for an entry of a dictionary having multiple dictionary blocks may be received, where the dictionary stores string values that are associated with corresponding value identifiers (IDs) for purposes of dictionary compression. One or more of the dictionary blocks can be selectively loaded into memory, and the loaded dictionary block(s) can be searched for the requested entry.

In some embodiments, a computer-readable storage medium may be provided to store structured data that can be used to load dictionary blocks for a dictionary into memory. The structured data may include multiple dictionary blocks for a dictionary that is adapted to store string values that are associated with corresponding value IDs for purposes of dictionary compression, where each of the dictionary blocks stores at least some of the string values. The structured data may also include one or more directory value ID blocks for a value ID directory that is adapted to include last value IDs for the dictionary blocks, where each of the directory value ID blocks stores at least some of the last value IDs. The structured data may also include one or more directory separator blocks for a separator directory that is adapted to include separators for the dictionary blocks, where each of the directory separator blocks stores at least some of the separators.

In some embodiments, a database system including one or more processors and memory may be provided. The memory may store one or more components for managing an in-memory column store (hereinafter, referred to as an “attribute engine”), which are adapted to provide access to a string dictionary; one or more components and/or structures for an in-memory vector implementation (hereinafter, referred to as a “basis”), which are adapted to store value block vectors, each of the value block vectors storing pointers to one or more value blocks for dictionary blocks of the string dictionary; and one or more components and/or structures for column store table persistence (hereinafter, referred to as a “unified table component”). The unified table component may include a dictionary storage component adapted to load one or more of the dictionary blocks, where each of the dictionary blocks comprises one or more value blocks; a value ID directory adapted to provide access to an entry of the dictionary given a value ID; a separator directory adapted to provide access to an entry of the dictionary given a separator (i.e., a string value or prefix thereof); and a large string handler adapted to provide access to large string values.

As described herein, a variety of other features and advantages can be incorporated into the technologies as desired.

The foregoing and other objects, features and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the architecture of an example in-memory database system that supports paged column dictionaries.

FIG. 2 a is a diagram illustrating data layout for the segments of a first example structure for storing a string value in a value block.

FIG. 2 b is a diagram illustrating data layout for the segments of a second example structure for storing a string value in a value block, which may be used for a large string value.

FIG. 3 is a diagram illustrating data layout for an example dictionary block and an example value block.

FIG. 4 is a diagram illustrating relationships between directory and dictionary pages.

FIG. 5 is a diagram illustrating the persistent data layout for example dictionary blocks and pages for a unified table of a paged column dictionary.

FIG. 6 is a flowchart of an example method for loading dictionary pages into memory.

FIG. 7 is a flowchart of another example method for loading dictionary pages into memory.

FIG. 8 is a diagram illustrating an example computing system in which some described embodiments can be implemented.

FIG. 9 is a diagram illustrating an example cloud computing environment that can be used in conjunction with the technologies described herein.

DETAILED DESCRIPTION

The following description is directed to techniques and solutions for implementing a paged column dictionary within an in-memory database system. For example, directories may be provided for a dictionary, where the directories are resident in memory and dictionary pages (i.e., value pages) are pageable.

A columnar table of an in-memory database system may include a delta part and a main part. The main part stores data for the table. The delta part receives changes to the table and stores these changes. A merge process may occasionally (e.g., periodically, or when the delta part reaches a certain size, etc.) update the main part with the delta part. For example, the merge process may copy a current main state of the main part and committed rows in the delta part into a new temporary state, sort the data for optimal compression, and overwrite the original main part. Committed rows may be removed from the delta part at the time of overwriting the original main part.

In some in-memory database systems, and, in particular, in some column-oriented database systems, dictionary compression may be used to reduce the in-memory and on-disk size of data. For example, for dictionary-compressed columns in a database, data and a dictionary associated with each column may be used to compress that column. When dictionary paging is not used, the data and dictionary are typically fully resident in memory. In contrast, when dictionary paging is used, pages of the dictionary are loaded into memory on demand when they are used. Either way, for a given column, each unique value may use a unique numeric value identifier (value ID), which may be associated with the unit value in the dictionary for the given column, so that the column data only stores the value IDs. Also, the corresponding unique values stored by the dictionary can be compressed using string prefix encoding or another form of encoding.

To access any data in a column store, when dictionary paging is not used, the entire contents of the dictionary and data for the column may be read into memory from disk as continuous vectors. However, the memory footprint of tables with a large number of columns and/or large columns can become fairly significant. In some cases, this memory footprint can be reduced at the cost of column data read performance, by defining specific columns to be represented both in-memory and on-disk as a collection of pages, rather than continuous vectors. The memory footprint of a paged column is reduced by requiring only an active subset of the column's data and dictionary pages in memory, which can be retrieved on demand from a persistent disk file. Furthermore, in order to reduce the memory footprint without affecting write performance, the main part of the column store may use a paged representation while the delta part does not use a paged representation.

In some example implementations, an in-memory data storage platform separates dictionary directory structures (constructed from directory pages) from dictionary data pages, such that the directory structures are resident in memory, but the dictionary data pages are pageable (and loaded into memory on demand). Thus, for a page-loadable column, the entire data for the column does not have to be loaded into memory to access a single value. Instead, only a small set of data is loaded into memory, including the dictionary value page that contains the multiple values as well as mapping information (in directory structures) used to retrieve the single value. The platform unifies handling of paged column dictionaries and regular (non-paged column) dictionaries, so that either or both types of dictionary can be used. Also, the platform unifies handling of large string values and other (non-large) string values. Large string values can be stored on their own pages and have logical pointers encoded into normal string values in a directory structure and dictionary data pages.

EXAMPLE 1 In-Memory Database System

FIG. 1 is a diagram illustrating the architecture of an example in-memory database system 100 that supports paged column dictionaries. The in-memory database system 100 may be implemented by various types of computing devices, such as computer servers, data storage systems, cloud computing resources, networking resources, etc. The in-memory database system 100 in FIG. 1 comprises an attribute engine 110, a unified table 130, and a basis 150, which can be organized as different component layers.

In general, the attribute engine 110 processes low-level database queries and provides mechanisms to query data from a table. For a column (attribute) of the table, the attribute engine 110 manages access to an index vector or other data representing values for the column, as well as access to a dictionary. The dictionary can be a paged dictionary, or the dictionary can be a non-paged dictionary, depending on the class of the attribute. A paged attribute is a class that facilitates access to data values and storage of data values in a paged column dictionary.

The attribute engine 110 may, e.g., form part of an in-memory column store such as the in-memory column store of the HANA platform (HANA is a database platform provided by SAP®) or another platform, so as to provide in-memory access to dictionary compressed column data. In FIG. 1 , the attribute engine 110 includes one or more components for a string dictionary 111. One or more components for the string dictionary 111 or other components of the attribute engine 110 may expose application programming interfaces (APIs) for, e.g., queries, such that data access via the attribute engine 110 is passed through the string dictionary 111. Alternatively, access to the string dictionary 111 can be exposed in some other way.

The component(s) for the string dictionary 111 represent a dictionary for variable-sized data, providing access and search functionality. The component(s) for the string dictionary 111 interact with components of the unified table 130. For this purpose, a component for the string dictionary 111 includes a handle to the main dictionary 131, which is a storage object of the unified table 130. For a non-paged, in-memory dictionary, this handle may be null, but the handle may have a valid, non-null value for a paged column dictionary.

As shown in FIG. 1 , the handle can be contained in a component for paged pages 112 (e.g., a component implementing an encoded string storage API) in the string dictionary 111. The component for paged pages 112 manages a container storing “physical pages” of a paged variable-sized attribute dictionary, and provides access to the strings encoded therein. For example, a physical page may contain one or more value blocks, where each value block consists of 16 prefix-encoded strings. With the handle, the component for paged pages 112 facilitates paged persistence through access to the storage object for the main dictionary 131. The dictionary 111 may also include other pages, such as default pages.

The attribute engine 110 may also contain a handle (not shown in FIG. 1 ) to in-memory transient vector(s), such as value block vectors, storing pointers to dictionary values, which are part of the basis 150.

The unified table 130 includes components that represent parts of a table. The unified table 130 may include a main dictionary 131 that stores variable length data (e.g., strings, fixed length strings, raw data, etc.). The main dictionary 131 is a storage component that serves as a point of interface between the attribute engine 110 and the unified table 130. The main dictionary 131 can contain a handle (not shown in FIG. 1 ) to the in-memory transient vector(s) (such as value block vectors) storing pointers to dictionary values, which are part of the basis 150. The main dictionary 131 can also contain a handle to a dictionary block handler (DBH), or multiple handles to DBHs.

For example, one DBH is a component that provides functionality to read from/write to a dictionary page, which includes value blocks. To provide this functionality, this DBH communicates with a component for the main page chain 141, which contains a chain of dictionary pages. A dictionary page represents a page of dictionary values, organized as value blocks. The component for the main page chain 141 can also provide functionality to manage dictionary pages, e.g., to load and pin a page, to allocate a new page, etc. Thus, main dictionary pages may be selectively loaded into memory as part of the main page chain 141.

As another example, another DBH is a component that manages in-memory transient structures used to access pages of a paged dictionary. One such DBH can manage construction of a value ID directory 132 and persistence of data for the value ID directory 132. Another such DBH can manage construction of a separator directory 133 and persistence of data for the separator directory 133.

The value ID directory 132 and separator directory 133 provide access to dictionary value block pages, given a value ID or search string, respectively. For example, the value ID directory 132 and separator directory 133 may be constructed from one or more directory page chains 140, each directory page containing value IDs or pointers to search strings. Details of the directory paging and construction of the value ID directory 132 and separator directory 133 will be explained in further detail below.

In some embodiments, the value ID directory 132 and separator directory 133 are loaded directly into memory. The separator directory 133 can include logical pointers, which are loaded into memory, to separator values represented on one or more other pages in a separator page chain (not shown in FIG. 1 , but explained below). In other embodiments, the unified table may include only one of the value ID directory 132 and separator directory 133. In still other embodiments, the paged dictionary values need not be accessed by a value ID or search string, and may be accessed by other means, e.g., an index value.

In the case of large strings (e.g., strings longer than a threshold size of 32K, or some other threshold size), such strings may be fragmented across multiple pages. The unified table 130 may include a large string handler 134 that provides, e.g., via a large string map, pointers to pages forming a large string page chain 142. For example, the large string handler 134 constructs a large string map, which is a transient object containing information about reconstructed large string values and their indices.

Alternatively, the unified table 130 may include one or more additional components or layers of components. For example, an intermediate component can include a handle to the main dictionary 131 and a handle to the main page chain 141.

The basis 150 provides generic structures such as value block vectors 151, 152, which include pointers to sorted dictionary values in value blocks of a dictionary page. For example, a value block vector 151, 152 stores pointers to (memory addresses of) the starts of the respective value blocks in a dictionary page. Thus, the basis 150 may provide value block vectors 151, 152 to search loaded pages from the main page chain 141 (i.e., the dictionary blocks of the main dictionary) for paged dictionary values. The process for searching paged dictionary values will be explained in further detail below.

During bootstrapping operations for a paged dictionary, directory pages are loaded into memory. The main dictionary 131 (dictionary storage object) may access the dictionary pages and populate the transient in-memory vectors to contain dictionary values or pointers to dictionary values on demand as dictionary pages are loaded.

During merge operations for a paged dictionary, main dictionary 131 (dictionary storage object) allocates new dictionary pages (via a DBH and the main page chain 141) and writes sorted and prefix-encoded dictionary values on pages in units of blocks.

In general, as used herein, the term “block” refers to a data structure in memory or persistent storage, and the term “page” refers to the data being loaded into memory. In some examples (e.g., for dictionary pages), a single block may be contained in a page, but there need not be a one-to-one relationship between blocks and pages. For example, multiple blocks may exist on one page (e.g., a page can include multiple value ID directory blocks and/or multiple blocks of logical pointers for separators, or a dictionary page can include multiple dictionary blocks). It should be apparent from the descriptions when a page contains a single block or multiple blocks.

EXAMPLE 2 Data Layouts for a Value Block

With a dictionary for a column of variable-sized values, the data for the dictionary can be organized (both in memory and on disk) as a series of value blocks. A dictionary block contains one or more value blocks.

FIG. 2 a is a diagram illustrating the data layout for segments of an example structure for storing a string value in a value block. A value block may contain a fixed number of prefix-encoded string values, e.g., 16 prefix-encoded string values. As an illustrative example, a value block may begin with the sequence “4 J o h n 4 3 s o n 2 3 n e s 0 6 M i l l e r . . .” to represent the sorted string values John, Johnson, Jones, Miller, . . .

A first segment of the string value may specify a prefix length 200. The prefix length specifies the number of characters that the string of the string value has in common with its predecessor string in the same value block. The first string value of a value block (i.e., the start of the value block) does not have a predecessor string, so its prefix length can be omitted. Alternatively, the first string value in a value block may include a first segment using, e.g., 0 or some other value indicating a first string value. Using the above illustrative example, for the second string value “Johnson,” ‘4’ indicates the number of characters in common with its predecessor string, i.e., “J o h n.” The segment for prefix length 200 can have a fixed length, e.g., one byte.

A second segment of the string value may specify a length of the remaining characters of the string 210, after the shared prefix. For example, in the illustrative example above, the first string value “John” contains the sequence “4 J o h n” in the second and third segments. The ‘4’ specifies the length of the string “John.” For the second string value “Johnson,” the ‘3’ specifies the length of the remaining characters “s o n.” Combined with the characters in common with its predecessor string value, “J o h n,” the resulting string is “Johnson.” The segment for remainder length 210 can have a variable length, e.g., one to six bytes using UTF-8 encoding.

A third segment can comprise the non-shared, remainder characters of the string 220. For the first string value in the above example, the characters “J o h n” result in the string “John.” For the second string value in the above example, the characters “s o n” result in the string “Johnson.” The segment with remainder characters of the string 220 has a length indicated by the second segment 210.

By using the above compression scheme, storage is reduced for the strings, providing savings in storage space.

FIG. 2 b is a diagram illustrating the data layout for segments of another example structure for a string value in a value block, which can be used for a large string. One or more of the prefix-encoded strings of a value block may be a large string. For a string value, a first segment may specify a prefix length 240, similar to the prefix length 200 described above. The segment for prefix length 240 can have a fixed length, e.g., one byte.

A second segment may specify a length of the characters of the current string value 250 in the value block. That is, the second segment may specify the length of the value block. The segment indicating the length of the block 250 can have a variable length of bytes encoded using UTF-8 encoding.

A third segment may include characters of the current string value 260 that are stored in the value block itself (i.e., a portion of the string after any prefix portion shared with the predecessor string). If a string is smaller than a threshold size, e.g., 32K or some other size, these remaining characters of the string can be stored “on page” (entirely in the value block). For a string larger than the threshold size, the string may be treated as a large string, which is fragmented between the third segment of the value block and one or more large string pages. In this case, a portion (of the remaining characters) of the large string is stored in the third segment. The remainder of the large string (after the prefix and the characters stored in the third segment) may be stored as one or more large string blocks (at one or more large string pages). The length of the third segment can be determined using the length of the value block 250, the total length of the string 290, and the number of logical pointer (LPs) 280. For example, the length of the third segment is given by the length of the value block—n bytes—m bytes, where n accounts for the bytes in the prefix length 240, the number of LPs 280 and the total length of the string 290, and where m is the number of bytes used by any LP(s) 270.

A fourth segment may specify zero or more LPs 270 to large string pages storing overflow data for a large string. Each LP 270 may be, e.g., 8 bytes in length. When the fourth segment includes multiple LPs, the LPs can be ordered from the end of the fourth segment to the start of the fourth segment.

A fifth segment may identify the number of LPs in the fourth segment 280. The fifth segment can be a 2-byte value that contains the number of LPs 280. For example, if the remainder of a large string is contained in two large string blocks, the number of LPs 280 would be 2.

A last segment may specify a total length of the string 290. The last segment can be a 6-byte value that contains the total length of the string 290. As noted, the block size 250, the total length of the string 290, and the number of LPs 280 may be used to determine the number of characters of the current string 260 in the third segment of the value block itself (that is, the length of the on-page part of the string).

Alternatively, a value block can have a different data layout. For example, one or more string values in a value block can be compressed using some other type of string compression.

EXAMPLE 3 Dictionary Block and Value Block Vector

FIG. 3 is a diagram illustrating data layout for an example dictionary block 300 and value block vector 350 (also called a value block pointer vector). The dictionary block 300 may include a variable dictionary block header 310, one or more value blocks 320, 321, 322, and corresponding offset values 330, 331, 332.

The variable dictionary block header 310 specifies how many value blocks 320, 321, 322 are included in the particular dictionary block 300 (i.e., the number of value blocks in the dictionary block 300, which can be one or more).

The dictionary block 300 then includes the one or more value blocks 320, 321, 322. The value blocks 320, 321, 322 can store prefix-encoded values. The data layout for the value blocks 320, 321, 322 can be the layout shown in FIG. 2 a , the layout shown in FIG. 2 b , or some other data layout.

Each value block 320, 321, 322 may be addressed by a corresponding offset value 330, 331, 332 for accessing the value block 320, 321, 322 in the dictionary block 300, where the offset value 330, 331, 332 specifies how many bytes from the beginning of the dictionary block 300 a particular value block 320, 321, 322 begins. The offset values 330, 331, 332 start from the end of the dictionary block 300. The offset values 330, 331, 332 can have a fixed length of, e.g., 4 bytes. For example, offset (1) 330 indicates the number of bytes from the beginning of the dictionary block 300 to the start of value block (1) 320, offset (2) 331 indicates the number of bytes from the beginning of the dictionary block 300 to the start of value block (2) 321, and so on. The dictionary block 300 is persistent (i.e., stored on fixed media/disk), but it can also be loaded into memory as a dictionary page.

In contrast, the value block vector 350 typically exists in memory only (making it transient). The value block vector 350 is an in-memory array that indexes the value blocks in the dictionary block 300. The value block vector 350 may include physical memory pointers (e.g., C++ character pointers) 351, 352, 353, 354, 355 to the starts for the value blocks, respectively. Each value block vector 350 may be associated with a dictionary block 300. When the value blocks and/or strings contained within the value blocks are loaded into memory, the offsets in the dictionary block 300 may be digested to determine the entries of the value block vector 350. Each entry in the value block vector 350 (i.e., char *) may reference the start of a particular value block in the dictionary block. For example, value block pointer 355 may point to the start of value block (N) 322. The dictionary block 300 may be searched using the value block vector 350.

The value block vector 350 (storing pointers to the start of each value block) and a large string map (storing or referencing large strings) may be associated on a per page basis with a corresponding dictionary block 300 in that page. These transient objects (value block vector 350 and large string map) may be created for every page that is loaded in memory. The large string map may store one or more pairs of values, each of the one or more pairs of values including an index and a large string value (or a pointer to a large string value).

A large string value may be materialized for one of the one or more pairs of values in the large string map. The materializing may include loading part of the large string value from a value block of the dictionary block (e.g., from a third segment of the data layout described with reference to FIG. 2 b ) and loading part of the large string value from one or more large string dictionary blocks (such as those referenced by LPs in the data layout described with reference to FIG. 2 b ). When a value block stores LPs to multiple large string dictionary blocks for a large string value, the large string pages for those large string blocks can be loaded into memory in parallel when materializing the large string value for the large string map, which can speed up the process of materializing the large string value.

In terms of data layout, a large string dictionary block may include a header and part of a large string value. Each large string dictionary page may contain a large string dictionary block.

The dictionary block 300, value block vector 350 and large string map can be used during load, access and merge operations. For example, for a load operation, a dictionary storage object (such as the main dictionary 131 described with reference to FIG. 1 ) pins a dictionary page containing the dictionary block 300 in memory. At this point, each entry of the value block vector 350 points to the start of a corresponding value block in the dictionary block 300. The dictionary storage object uses large string dictionary pages, if any, to materialize large string values, if any, for the large string map. For any large string value, the large string map stores a pair of an index value and large string value (or pointer to the large string value).

As another example, for an access (get) operation, the value block vector 350 is used to look up a dictionary value in the dictionary block 300 loaded into memory. If the value is a large string value, the value is looked up in the large string map and returned (or, a pointer to the string value in the large string map is returned). If the value is not a large string value, the value is looked up in the dictionary block 300 using the value block vector 350, decoded and returned (or, a pointer to the value block is returned).

As another example, for a merge operation, data is written, one value block at a time, to an allocated page containing a dictionary block 300. As appropriate, overflow data is written to one or more large string dictionary blocks in large string dictionary pages. The value block vector 350 is updated accordingly. Allocated pages for main dictionary blocks may be pinned temporarily, when a string contained therein is being accessed, then unpinned afterwards so as to allow them to be paged to disk.

In some example implementations, a string dictionary can be a loaded dictionary or a paged dictionary. A paged dictionary includes multiple dictionary pages having dictionary blocks, such as the dictionary block 300. The paged dictionary can be arranged such that data is paged to and from disk on demand. Transient data structures (such as the value block vector 350 and a large string map) are associated with a dictionary page (and hence a dictionary block). Structures for page lookup operations (such as a value ID directory and separator directory, as described below) are associated with the entire paged dictionary. In contrast, for a loaded dictionary, data values (e.g., regular length string values) are fully resident in memory, and a transient data structure (value block vector) is associated with the entire dictionary. For the loaded dictionary, any large string can be materialized on demand.

EXAMPLE 4 Directory and Dictionary Pages for Unified Table

FIG. 4 is a diagram illustrating relationships between directory and dictionary pages for a unified table, such as the unified table 130 of FIG. 1 or another unified table. The dictionary pages contain, for example, blocks of data loaded from persistent storage into memory.

Dictionary storage objects may include objects such as a value ID directory 420, a separator directory 440, and transient objects such as a value block vector and large string map 450. To support paging in a main dictionary, directory/look-up information for each main dictionary may be stored to ensure that only relevant dictionary blocks are loaded into memory. In order to load the relevant main dictionary blocks, the following information may be stored for each dictionary block: a last value ID for the dictionary block and a separator (discerning prefix) of the uncompressed last value in the dictionary block. In particular, the value ID directory 420 and separator directory 440 are constructed using such information, and then used to determine which dictionary value blocks to load into memory.

The value ID directory 420 may include the last value IDs for main dictionary string blocks. In the value ID directory 420, the last value ID for a given main dictionary string block is associated with a page index for the page containing that main dictionary string block in the main dictionary page chain. For example, the entry at index 0 of the value ID directory 420 contains the last value ID of a string on page 0 in the main dictionary page chain 410.

The separator directory 440 may include LPs to separators for the main dictionary string blocks. In the separator directory 440, the separator for a given main dictionary string block is associated with a page index for the page containing that main dictionary string block in the main dictionary page chain. For example, the entry at index 0 of the separator directory 440 contains the separator for the last string (or part thereof) stored on page 0 in the main dictionary page chain 410. Alternatively, instead of storing LPs to separators, the separator directory 440 may include uncompressed last value separators for the main dictionary string blocks.

The value block vector, e.g., as described above with reference to FIG. 3 , stores character pointers (char *) to the starts of the respective value blocks of a dictionary block contained in a dictionary page of the main dictionary page chain 410. That is, the value block vector is an array of physical memory pointers to each value block in the dictionary page. The large string map, e.g., as described above with reference to FIG. 3 , stores or references materialized large string values. The value block vector and large string map 450 are associated with a particular dictionary block/dictionary page, and they can be created for each dictionary page that is loaded in memory.

Various page chains may also be used to support paged dictionaries. The page chains include blocks of data, which can be loaded into memory. FIG. 4 shows a main dictionary page chain 410, a large string page chain 460, a main dictionary directory separator page chain 470, and a main dictionary directory value ID/separator logical pointer LP page chain 430.

The main dictionary page chain 410 may include pages storing main dictionary blocks. The dictionary blocks are selectively loaded based on the last value ID or separator of the uncompressed last value for the respective dictionary blocks. In some example implementations, one main dictionary page chain 410 may be loaded per column (attribute), which includes dictionary blocks for the dictionary for that column.

The large string page chain 460 may include large string pages for the large string map. In the example implementations, one large string page chain 460 may be loaded per table, so the large string page chain 460 potentially includes large string pages for dictionaries associated with different columns (attributes) of the table.

The main dictionary directory value ID/separator LP page chain 430 may include pages storing main dictionary directory value ID blocks (for the value ID directory) and/or main dictionary directory separator LP blocks (for the separator directory 440). A single page can include one or more value ID blocks and one or more separator LP blocks. A main dictionary directory value ID block may store last value IDs of dictionary blocks, where the last value IDs are typically fixed-length values. A main dictionary directory separator LP block may store LPs to some of the separators for dictionary blocks, where the LPs are typically fixed-length values. In the example implementations, one main dictionary directory value ID/separator LP page chain 430 may be loaded per table, so the directory value ID/separator LP page chain 430 potentially includes value ID blocks and separator LP blocks for dictionaries associated with different columns (attributes) of the table.

The main dictionary directory separator page chain 470 may include main dictionary directory separator pages (for a separator directory). A main dictionary directory separator page may include one or more separators for the uncompressed last values of dictionary blocks, where the separators are typically variable-length values. In the example implementations, one main dictionary directory separator page chain 470 may be loaded per table, so the separator page chain 470 potentially includes separator pages for dictionaries associated with different columns (attributes) of the table.

Alternatively, one of each of the page chains can be associated with a given column. However, the page chains are not limited as such. For example, the page chains are not limited to either column or table. For example, one main dictionary page chain may be loaded per table. Alternatively, more than one main dictionary page chain may be loaded per column.

In the previous examples in this section, the value ID directory includes last value IDs, and the separator directory includes separators for last string values. Alternatively, a value ID directory and separator directory can include (store or reference) other types of values. For example, the value ID directory includes first value IDs of string values of dictionary blocks, and the separator directory includes separators for first string values of dictionary blocks. The first value IDs and separators (for first string values) can be used to determine which dictionary block to load into memory, which dictionary block to search for a value, and so on.

EXAMPLE 5 Persistent Data Layouts for Paged Column Dictionary

FIG. 5 is a diagram illustrating data layout for example dictionary blocks and pages for a paged column dictionary. FIG. 5 shows a main dictionary directory value ID block 510, a main dictionary directory separator block 520, a main dictionary directory separator LP block 530, two main dictionary blocks 540, 550, and two large string dictionary blocks 560, 570.

FIG. 5 shows persistent data used to implement a value ID directory and a separator directory. Directory information may be stored as value ID directory blocks, separator directory blocks, and/or separator LP directory blocks. For example, the value ID directory blocks and separator LP directory blocks are blocks of fixed length data, and the separator blocks are blocks of variable length data. For value ID directory blocks and separator LP directory blocks, multiple blocks may be stored on the same page in a page chain. Further, for separator directory blocks, multiple blocks may be stored on the same page in a page chain.

As explained with reference to FIG. 4 , a value ID directory may map the value ID (e.g., a last value ID) of each main dictionary block to the page chain index of the page that contains that main dictionary block. Persistent data for the value ID directory may be stored as one or more main dictionary directory value ID blocks, which store last value IDs for main dictionary blocks. In FIG. 5 , the main dictionary directory value ID block 510 contains a value ID (e.g., last value ID) of each of several main dictionary blocks 540, 550 of the main dictionary. An entry of the main dictionary directory value ID block 510 stores a value ID (e.g., last value ID) for a main dictionary block. For example, VID 1 of the main dictionary directory value ID block 510 contains the last value ID associated with a final string value represented in the main dictionary block 540 in the main dictionary page chain. VID 2 of the main dictionary directory value ID block 510 contains the last value ID associated with a final string value represented in the main dictionary block 550 in the main dictionary page chain. The value ID directory allows the correct main dictionary block page to be loaded from disk into memory given a value ID. The value ID directory may be persisted in the one or more main dictionary directory value ID blocks 510. Main dictionary directory value ID blocks 510 may be contained in main dictionary directory pages, which may contain multiple such blocks.

As explained with reference to FIG. 4 , a separator directory may map a discerning string prefix (e.g., the last discerning string prefix) of each main dictionary block to the page chain index of the page that contains that main dictionary block. Persistent data for the separator directory may be stored as one or more separator blocks, which store separators (discerning prefixes) for string values (e.g., the last string values) of the respective main dictionary blocks, and one or more separator LP blocks, which store LPs to the separators in the separator blocks. In FIG. 5 , the main dictionary directory separator block 520 contains separators (e.g., uncompressed last value strings, or parts thereof) of each of several main dictionary block 540, 550 of the main dictionary. The uncompressed value strings can be stored in value block format, one string per value block. An entry of the main dictionary directory separator block 520 stores a separator, or part of a separator, for a main dictionary block. For example, SEPARATOR VAL 1 of the main dictionary directory separator block 520 contains the last value string (or part of the last string value) for the main dictionary block 540 in the main dictionary page chain. SEPARATOR VAL 2 of the main dictionary directory separator block 520 contains the last value string (or part of the last string value) for the main dictionary block 550 in the main dictionary page chain. The separator directory allows the correct main dictionary block page to be loaded from disk into memory given a separator. Main dictionary directory separator blocks 520 may be contained in main dictionary separator pages, which may contain multiple separator strings.

The one or more main dictionary directory separator LP blocks contain logical pointers (LPs) to the separators of the main dictionary directory separator blocks. In FIG. 5 , the main dictionary directory separator LP block 530 stores LPs to separators in the main dictionary directory separator block 520. For example, LP 1 of main dictionary directory separator LP block 530 is a LP to SEPARATOR VAL 1 of main dictionary directory separator block 520. Main dictionary directory separator LP blocks may be contained in main dictionary directory pages, which may contain multiple such blocks.

If a separator string value is a large string (e.g., greater than a threshold size such as 32K or some other size), an entry of the directory separator block 520 may contain the first 32K of the string and one or more LPs to one or more large string dictionary blocks 560, 570 containing the remainder of the string. For example, LP 1 of main dictionary directory separator block 520 may reference large string dictionary block 560 for part of SEPARATOR VAL 2, and LP 2 of main dictionary directory separator page 520 may reference large string dictionary block 570 for the rest of SEPARATOR VAL 2.

EXAMPLE 6 Loading Dictionary Pages and Other Operations

FIG. 6 is a flowchart of an example method for loading dictionary pages into memory.

To start, a request for an entry of a dictionary is received (610). In general, the dictionary stores string values that are associated with corresponding value IDs for purposes of dictionary compression. The dictionary may have multiple dictionary blocks. The request may contain a value ID or a value string. For example, the request can be to find a string value for a particular value ID. Or, the request can be to find a string value after a particular value ID. Or, the request can be to find a value ID for a particular string value.

Suppose the column data of a table contains entries having a value ID corresponding to a variable length string in the dictionary. To retrieve a value corresponding to a value ID or value ID corresponding to a string, the dictionary determines which dictionary block to load based on the value ID or value string of the request.

Once the correct dictionary block is identified, if the dictionary block is not already loaded into memory (615), the dictionary block is loaded into memory (620). This allows for on-demand page management of the in-memory database, since only those dictionary blocks that are used are loaded into memory. If the identified dictionary block has already been loaded into memory, the loading (620) is skipped. Thus, the identified dictionary block is selectively loaded into memory.

The dictionary block that has been loaded into memory is then searched (630). Since the appropriate dictionary block is loaded into memory, the dictionary can be searched for the requested variable length string.

FIG. 7 is a flowchart of another example method for loading dictionary pages into memory.

A request to access a column is received (710). For example, the request to access the column can be an initial request for an entry of the column, which is represented in the dictionary, or it can be some other type of request. Since the data for the column contains value IDs, values corresponding to the value IDs need to be retrieved. However, instead of retrieving values for all of the value IDs, only those values needed may be loaded. To determine which values are needed, directory structures are constructed.

A value ID directory may be constructed in memory at the time of column access (720). The value ID directory may be constructed by loading one or more main dictionary directory value ID blocks, which contain value IDs for the multiple dictionary blocks. The directory entries for the value ID directory may be fully materialized (i.e., loaded into memory) during loading, resulting in a two-dimensional array that maps value IDs (e.g., last value IDs) to page indices for the pages that contain the corresponding dictionary blocks. Alternatively, the directory value ID blocks may be scanned at runtime when handling a load or access operation. The value ID directory may include value IDs (e.g., last value IDs) for the multiple dictionary blocks, and each of the value IDs may be mapped to an index of one of the multiple dictionary blocks that includes a string value for that value ID.

A separator directory may also be constructed in memory at the time of column access (730). The separator directory may be constructed by loading one or more main dictionary directory separator pages (which contain separators for the multiple dictionary blocks) and one or more separator LP pages (which contain LPs to the separators for the multiple dictionary blocks). The directory entries for the separator directory may be LPs that point to data resident on the separator pages, and the main dictionary directory separator pages can be pinned for the lifespan of the dictionary. The separator directory may include (store or reference) separators for the multiple dictionary blocks, and each of the separators may be mapped to an index of one of the multiple dictionary blocks that includes a string value for that separator.

If a separator value is a large string, the separator value may be fully materialized. In other words, the large string dictionary pages will be loaded to fully materialize this value.

One or more dictionary entry requests may then be iteratively processed (738). In FIG. 7 , operations 740 to 770 illustrate processing for a single request for an entry of the dictionary. The operations can be repeated for one or more other requests.

A request for an entry of the dictionary is received (740). Again, the dictionary generally stores string values that are associated with corresponding value IDs for purposes of dictionary compression, and the dictionary may have multiple dictionary blocks. The request may contain a value ID or a value string.

The value ID directory or separator directory may be searched to identify a dictionary block (i.e., determine the index of the page containing the relevant main dictionary block) (750). For example, the value ID directory may be searched using a value ID for the request (that is, comparing the value ID to the value IDs in the value ID directory) to determine an index of the one of the multiple dictionary blocks that is to be loaded into memory. Or, the separator directory may be searched using a string value for the request to determine the index of the one of the multiple dictionary blocks that is to be loaded into memory. The search of the value ID directory or separator directory can be a binary search or other type of search.

If the dictionary block is not already loaded into memory (755), the dictionary block may be loaded into memory (760) (i.e., as a main dictionary page). A value block vector (and large string map) associated with the main dictionary page may also be constructed.

The dictionary block that has been loaded into memory may be searched (770). For example, the value block vector associated with the dictionary page for the block may be binary searched using a value ID for the request to find the requested value. (The string values in the value blocks of the dictionary page for the paged dictionary can be accessed using the physical memory pointers in the value block vector, like the string values for a non-paged dictionary.)

If the requested value is a large string and has not been materialized, the large string pages storing overflow data may be loaded to build the large string on demand. The string may be added to the large string map associated with the page. Alternatively, large string values in the large string map are materialized when the dictionary block is loaded into memory.

When a second request for an entry of the dictionary having multiple dictionary blocks is received, the dictionary block that includes the entry is selectively loaded into memory (the value ID directory and separator directory having already been constructed). It may be determined that the dictionary block that includes the entry for the second request has already been loaded into memory. Thus, the dictionary block need not be reloaded into memory. The loaded (i.e., previously loaded) dictionary block may be searched for the entry.

When a loaded page is removed from memory, the associated value block vector can be cleared and any entries in the associated large string map can be de-allocated.

For a merge operation, data can be written to main dictionary blocks and large string pages as required. When a page containing a main dictionary block is full, the corresponding last value IDs and uncompressed last value separators may be written to the value ID directory and the separator directory, respectively. The main dictionary directory separator page(s) can always be pinned for the lifespan of the dictionary.

In some example implementations, paged dictionaries are used for columns of main storage (main memory), but not for delta stores. The paged dictionaries are created or updated upon delta merge operations, but remain read-only between delta merge operations. For a delta merge operation, old dictionaries can be replaced with new dictionaries. The old dictionaries can be garbage collected after a successful merge operation.

EXAMPLE 7 Computing Systems

FIG. 8 depicts a generalized example of a suitable computing system 800 in which the described innovations may be implemented. The computing system 800 is not intended to suggest any limitation as to scope of use or functionality, as the innovations may be implemented in diverse general-purpose or special-purpose computing systems.

With reference to FIG. 8 , the computing system 800 includes one or more processing units 810, 815 and memory 820, 825. In FIG. 8 , this basic configuration 830 is included within a dashed line. The processing units 810, 815 execute computer-executable instructions. A processing unit can be a general-purpose central processing unit (CPU), processor in an application-specific integrated circuit (ASIC) or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example, FIG. 8 shows a central processing unit 810 as well as a graphics processing unit or co-processing unit 815. The tangible memory 820, 825 may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two, accessible by the processing unit(s). The memory 820, 825 stores software 880 implementing one or more innovations described herein, in the form of computer-executable instructions suitable for execution by the processing unit(s).

A computing system may have additional features. For example, the computing system 800 includes storage 840, one or more input devices 850, one or more output devices 860, and one or more communication connections 870. An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing system 800. Typically, operating system software (not shown) provides an operating environment for other software executing in the computing system 800, and coordinates activities of the components of the computing system 800.

The tangible storage 840 (also called computer-readable storage) may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information in a non-transitory way and which can be accessed within the computing system 800. The storage 840 stores instructions for the software 880 implementing one or more innovations described herein.

The input device(s) 850 may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing system 800. The output device(s) 860 may be a display, printer, speaker, CD-writer, or another device that provides output from the computing system 800.

The communication connection(s) 870 enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media can use an electrical, optical, RF, or other carrier.

The innovations can be described in the general context of computer-executable instructions (also called machine-readable instructions), such as those included in program modules (also called computer program product), being executed in a computing system on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules may be executed within a local or distributed computing system.

The disclosed methods can also be implemented using specialized computing hardware configured to perform any of the disclosed methods. For example, the disclosed methods can be implemented by an integrated circuit specially designed or configured to implement any of the disclosed methods (e.g., an ASIC such as an ASIC digital signal process unit, a graphics processing unit, or a programmable logic device such as a field programmable gate array).

The terms “system” and “device” are used interchangeably herein. Unless the context clearly indicates otherwise, neither term implies any limitation on a type of computing system or computing device. In general, a computing system or computing device can be local or distributed, and can include any combination of special-purpose hardware and/or general-purpose hardware with software implementing the functionality described herein.

For the sake of presentation, the detailed description uses terms like “determine” and “use” to describe computer operations in a computing system. These terms are high-level abstractions for operations performed by a computer, and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.

EXAMPLE 8 Cloud Computing Environment

FIG. 9 depicts an example cloud computing environment 900 in which the described technologies can be implemented. The cloud computing environment 900 comprises cloud computing services 910. The cloud computing services 910 can comprise various types of cloud computing resources, such as computer servers, data storage repositories, networking resources, etc. The cloud computing services 910 can be centrally located (e.g., provided by a data center of a business or organization) or distributed (e.g., provided by various computing resources located at different locations, such as different data centers and/or located in different cities or countries).

The cloud computing services 910 are utilized by various types of computing devices (e.g., client computing devices), such as computing devices 920, 922, and 924. For example, the computing devices (e.g., 920, 922, and 924) can be computers (e.g., desktop or laptop computers), mobile devices (e.g., tablet computers or smart phones), or other types of computing devices. For example, the computing devices (e.g., 920, 922, and 924) can utilize the cloud computing services 910 to perform computing operators (e.g., data processing, data storage, and the like).

EXAMPLE 9 Implementation Choices

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed methods can be used in conjunction with other methods.

Any of the disclosed methods can be implemented as computer-executable instructions or a computer program product stored on one or more computer-readable storage media and executed on a computing device (e.g., any available computing device, including smart phones or other mobile devices that include computing hardware). Computer-readable storage media are any available tangible media that can be accessed within a computing environment (e.g., one or more optical media discs such as DVD or CD, volatile memory components (such as DRAM or SRAM), or nonvolatile memory components (such as flash memory or hard drives)). By way of example and with reference to FIG. 8 , computer-readable storage media include memory 820, 825, storage 840, and combinations of any of the above. The term computer-readable storage media does not include signals and carrier waves. In addition, the term computer-readable storage media does not include communication connections (e.g., 870).

Any of the computer-executable instructions for implementing the disclosed techniques as well as any data created and used during implementation of the disclosed embodiments can be stored on one or more computer-readable storage media. The computer-executable instructions can be part of, for example, a dedicated software application or a software application that is accessed or downloaded via a web browser or other software application (such as a remote computing application). Such software can be executed, for example, on a single local computer (e.g., any suitable commercially available computer) or in a network environment (e.g., via the Internet, a wide-area network, a local-area network, a client-server network (such as a cloud computing network), or other such network) using one or more network computers.

For clarity, only certain selected aspects of the software-based implementations are described. Other details that are well known in the art are omitted. For example, it should be understood that the disclosed technology is not limited to any specific computer language or program. For instance, the disclosed technology can be implemented by software written in C++, Java, Perl, JavaScript, Adobe Flash, or any other suitable programming language. Likewise, the disclosed technology is not limited to any particular computer or type of hardware. Certain details of suitable computers and hardware are well known and need not be set forth in detail in this disclosure.

Furthermore, any of the software-based embodiments (comprising, for example, computer-executable instructions for causing a computer to perform any of the disclosed methods) can be uploaded, downloaded, or remotely accessed through a suitable communication means. Such suitable communication means include, for example, the Internet, the World Wide Web, an intranet, software applications, cable (including fiber optic cable), magnetic communications, electromagnetic communications (including RF, microwave, and infrared communications), electronic communications, or other such communication means.

The disclosed methods, apparatus, and systems should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed methods, apparatus, and systems are not limited to any specific aspect or feature or combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present or problems be solved.

Various alternatives to the examples described herein are possible. For example, some of the methods described herein can be altered by changing the ordering of the acts described, or by splitting, repeating, or omitting certain acts, etc. The various aspects of the disclosed technology can be used in combination or separately. Different embodiments use one or more of the described innovations. Some of the innovations described herein address one or more of the problems noted in the background. Typically, a given technique/tool does not solve all such problems.

The technologies from any example can be combined with the technologies described in any one or more of the other examples. In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only examples of the disclosed technology and should not be taken as limiting the scope of the disclosed technology. Rather, the scope of the disclosed technology includes what is covered by the scope and spirit of the following claims. 

We claim:
 1. A method comprising: constructing a value identifier (ID) directory in memory for a dictionary having multiple dictionary blocks, each dictionary block comprising a plurality of value blocks, each value block comprising a value ID and comprising a plurality of string values; receiving a request for an entry of the dictionary having multiple dictionary blocks, wherein the dictionary stores string values associated with corresponding value IDs for dictionary compression; searching the value ID directory using a value ID associated with the request; selecting a dictionary block of the multiple dictionary blocks as responsive to the request by comparing the value ID to a plurality of value IDs of the value identifier directory to identify the dictionary block as comprising dictionary information for the value ID; loading the selected dictionary block from persistent storage into memory in response to determining that the value ID is present in the dictionary block; searching the selected dictionary block that has been loaded into memory for the value ID; identifying a string value corresponding to the value ID; and returning the string value in response to the request.
 2. The method of claim 1, wherein the value ID directory is constructed using one or more directory blocks that contain last values IDs for the multiple dictionary blocks, wherein the searching the selected dictionary block comprises locating the value block at least in part using at least one of the last value IDs.
 3. The method of claim 1, wherein each value block comprises a fixed number of string values.
 4. The method of claim 1, wherein a string value in a given value block of the plurality of value blocks is represented using segments that comprise: when the string value does not start the given value block, a first segment specifying a prefix length of a string; a second segment specifying a remainder length of the string; and a third segment comprising a remainder string of the string; wherein the string value is determined from at least a portion of the segments.
 5. The method of claim 1, wherein a string value in a given value block of the plurality of value blocks is represented using segments that comprise: when the string value does not start the given value block, a first segment specifying a prefix length of a string; a second segment specifying a length of the segments of the given value block; a third segment comprising a portion of the string; a fourth segment specifying zero or more logical pointers to large string pages; a fifth segment specifying number of the logical pointers in the fourth segment; and a sixth segment specifying a total length of the string; wherein the string value is determined from at least a portion of the segments.
 6. The method of claim 1, wherein each of the multiple dictionary blocks further comprises: a variable dictionary block header comprising information that indicates a number of value blocks in the dictionary block; and offset values for accessing the plurality value blocks in the dictionary block, each of the offset values specifying how many bytes from a beginning of the dictionary block a corresponding one of the one or more value blocks begins, wherein the searching the selected dictionary block comprises locating the value block at least in part using at least one of the offset values.
 7. The method of claim 1, further comprising: constructing a value block vector associated with the dictionary block that has been loaded into memory, wherein the value block vector stores logical pointers, each of the logical pointers referencing a start of a value block of the dictionary block, and wherein the searching the dictionary block uses the value block vector.
 8. The method of claim 1, further comprising: constructing a large string map associated with the dictionary block that has been loaded into memory, wherein the large string map stores one or more pairs of values, each of the one or more pairs of values comprising (a) an index and (b) a large string value or pointer to the large string value, wherein the identifying the string value comprises identifying the large string value or pointer using the index.
 9. The method of claim 8, wherein the constructing the large string map includes, for one of the one or more pairs of values in the large string map, materializing the large string value, including: loading part of the large string value from a value block of the dictionary block that has been loaded into memory; and loading part of the large string value from one or more large string dictionary blocks, each of the one or more large string dictionary blocks comprising: a header; and one or more parts of large string values, and wherein the returning the string value comprises returning the large string value.
 10. The method of claim 1, further comprising: instantiating a value block vector for the selected dictionary block, the value block vector storing pointers to one or more value blocks of the selected dictionary block, wherein the searching the selected dictionary block comprises identifying and following a pointer of the pointers corresponding to the value ID.
 11. The method of claim 1, wherein the request is a first request, the method further comprising: constructing a separator directory in memory, wherein the separator directory comprises separators for the multiple dictionary blocks, and wherein each of the separators is mapped to an index of one of the multiple dictionary blocks that includes a string value for that separator; receiving a second request for a second entry of the dictionary, the request comprising a string value; identifying the second entry by comparing the string value with the separator directory; and returning the second entry in response to the second request.
 12. One or more computer-readable storage media storing structured data usable by one or more processors to load dictionary blocks for a dictionary into memory, the structured data stored on the computer-readable storage media comprising computer executable instruction that, when executed by a processor of a computer, cause the computer to perform operations comprising: defining multiple dictionary blocks for a dictionary comprising string values associated with corresponding value identifiers (IDs) for dictionary compression, each of the multiple dictionary blocks storing at least some of the string values; defining one or more directory value ID blocks for a value ID directory comprising value IDs for the multiple dictionary blocks, each of the one or more directory value ID blocks storing at least some of the last value IDs; receiving a request for an entry of the dictionary having multiple dictionary blocks, wherein the dictionary stores string values associated with corresponding value IDs for dictionary compression; searching the value ID directory using a value ID associated with the request; selecting a dictionary block of the multiple dictionary blocks as responsive to the request by comparing the value ID to a plurality of value IDs of the value identifier directory to identify the dictionary block as comprising dictionary information for the value ID; loading the selected dictionary block from persistent storage into memory in response to determining that the value ID is present in the dictionary block; searching the selected dictionary block that has been loaded into memory for the value ID; identifying a string value corresponding to the value ID; and returning the string value in response to the request.
 13. The one or more computer-readable storage media of claim 12, the operations further comprising: constructing a separator directory in memory, wherein the separator directory comprises separators for the multiple dictionary blocks, and wherein each of the separators is mapped to an index of one of the multiple dictionary blocks that includes a string value for that separator; receiving a second request for a second entry of the dictionary, the request comprising a string value; searching the separator directory using the string value of the request to determine an index of the one of the multiple dictionary blocks that is to be loaded into memory; and loading the one of the multiple dictionary blocks into memory; searching the one of the multiple dictionary blocks loaded into memory for the second entry; and returning the second entry in response to the second request.
 14. The one or more computer-readable storage media of claim 13, wherein the structured data stored on the one or more computer-readable storage media further comprises: one or more separator logical pointer (LP) blocks for the separator directory, each of the one or more separator LP blocks storing LPs to at least some of the separators, wherein the searching the separatory directory comprises identifying a separator LP block of the one or more separator LP blocks corresponding to the string.
 15. A system comprising: one or more processors; a memory coupled to the one or more processors; and one or more computer-readable media storing computer executable instructions that, when executed by the one or more processors, cause the system to perform operations comprising: constructing a value identifier (ID) directory in memory for a dictionary having multiple dictionary blocks, each dictionary block comprising a plurality of value blocks, each value block comprising a value ID and comprising a plurality of string values; receiving a request for an entry of the dictionary having multiple dictionary blocks, wherein the dictionary stores string values associated with corresponding value IDs for dictionary compression; searching the value ID directory using a value ID associated with the request; selecting a dictionary block of the multiple dictionary blocks as responsive to the request by comparing the value ID to a plurality of value IDs of the value identifier directory to identify the dictionary block as comprising dictionary information for the value ID; loading the selected dictionary block from persistent storage into memory in response to determining that the value ID is present in the dictionary block; searching the selected dictionary block that has been loaded into memory for the value ID; identifying a string value corresponding to the value ID; and returning the string value in response to the request.
 16. The system of claim 15, the operations further comprising: searching the value ID directory for a last value ID of one of the multiple dictionary blocks that contains the value ID provided in the request.
 17. The system of claim 15, wherein the request is a first request, the operations further comprising: constructing a separator directory in memory, wherein the separator directory comprises separators for the multiple dictionary blocks, and wherein each of the separators is mapped to an index of one of the multiple dictionary blocks that includes a string value for that separator; receiving a second request for a second entry of the dictionary, the request comprising a string value; searching the separator directory using the string value of the request to determine an index of the one of the multiple dictionary blocks that is to be loaded into memory; and loading the one of the multiple dictionary blocks into memory; searching the one of the multiple dictionary blocks loaded into memory for the second entry; and returning the second entry in response to the second request.
 18. The system of claim 17, wherein the separator directory comprises one or more separator logical pointer (LP) blocks, each of the one or more separator LP blocks storing LPs to at least some of the separators, wherein the searching the separatory directory comprises identifying a separator LP block of the one or more separator LP blocks corresponding to the string.
 19. The system of claim 15, the operations further comprising: constructing a large string map associated with the dictionary block that has been loaded into memory, wherein the large string map stores one or more pairs of values, each of the one or more pairs of values comprising (a) an index and (b) a large string value or pointer to the large string value, wherein the identifying the string value comprises identifying the large string value or pointer using the index.
 20. The system of claim 15, the operations further comprising: instantiating a value block vector for the selected dictionary block, the value block vector storing pointers to one or more value blocks of the selected dictionary block, wherein the searching the selected dictionary block comprises identifying and following a pointer of the pointers corresponding to the value ID. 